A modern car may include hundreds of electronic sensors and actuators such as driver compartment temperature sensors and seat belt alarms that are polled and controlled by one or more central units (CUs). In common polling and control arrangements, the sensors and actuators are connected via hundreds of cables that are bundled in cable harnesses with a combined length that can reach 1-4 km. The growing cost and expanding utilization of copper produced by such arrangements presents an important design issue for the ongoing development of automotive vehicles as well as for industrial manufacturing systems. The expanding use of electronically controlled end devices now tests the practical limits of any practical wired interconnection arrangement.
A “fieldbus” wiring system targeted for industrial environments was developed in the late 1980s to share data connections in hard-wired sensor networks to reduce the number of copper interconnections. A fieldbus system is generally used with a wired local area network of sensors and actuators, and follows one of a variety of standards for real-time distributed control. Fieldbus approaches were originally developed to replace the RS-232 serial interface that requires a terminating communication element at each end of a twisted wire pair that might carry 4 to 20 mA of dc current. The RS-232 serial interface does not easily accommodate daisy chain- or ring-type communication structures, and accordingly requires a substantial amount of wiring to implement a complex or widely dispersed network.
An advantage of a wired network such as a fieldbus wiring system is its robustness against external interferences, particularly if shielded cables are used, which leads to very low bit error rates (BERs). However, communication reliability depends on the physical integrity of the cables which make a fieldbus arrangement prone to mechanical interruptions.
Two major disadvantages of wired networks, especially for automotive applications, are limited flexibility of mounting and deploying sensor devices because of the necessary cable routing. In addition, increased weight and costs are added to a vehicle by the extensive number and length of cabling and interconnections generally required to couple sensor units with a central unit.
The use of wireless communication techniques in applications that span automotive and industrial systems, office automation, security and alarm systems, and environmental monitoring and surveillance would introduce new design flexibilities as well as replace a substantial amount of copper cabling. Even a partial replacement of cables with wireless data transmission would be an advantageous enhancement to a fieldbus approach.
The performance of wireless sensor networks is often limited by contention for time or frequency slots by a large number of simultaneously communicating sensors. The quantity of data that can be transmitted by an individual sensor ranges from a single bit, such as a bit associated with a contact closure, to a larger file, such as a download of a sensor history file. There is also a wide range of acceptable latencies for data communication. For example, a sensed temperature in an interior compartment of an automobile can be easily deferred for a number of seconds, whereas sensing the motion of an adjustable mirror in an automobile is a time-critical event that can be readily detected by a human operator if it is delayed. Consequently, contention by multiple sensor nodes for common-use timeslots has been an ongoing problem in wireless sensor networks.
A further disadvantage of the use of a WSN (wireless sensor network) in a harsh industrial or automotive environment is its susceptibility to radio noise and interference, which increases the bit error rate. When using a conventional WSN architecture, there are two opposing design options with necessary tradeoffs. One is to use a conventional star topology (as illustrated, for example, in FIG. 1) which carries less traffic, resulting in fewer radio collisions and shorter delays. However, redundant transmission paths are not provided, which results in a higher BER if the network nodes are separated by any substantial distance. Another design option is to use a conventional wireless ad hoc multi-hop network topology such as a cluster-tree or mesh topology. This produces shorter hop-to-hop transmission paths, which leads to less hop-to-hop bit errors. However, more network traffic results due to message forwarding, which leads to a higher probability of radio collisions and longer communication delays. It also makes ad hoc routing less predictable, which is less suitable for industrial or automotive applications.
A conventional networking solution to overcome these limitations in a plain star topology WSN is to dynamically increase the transmission power of the nodes that lie on the physical periphery of the network. But this results in two drawbacks. One is that some of the wireless sensor nodes require more energy, and a second is that the higher transmission power increases the radio interference range among the nodes.
Another solution to overcome these limitations is to structure the network with wired-to-wireless gateways. Although some conventional hybrid networks use wired-to-wireless gateways, they simply use fixed gateways that each communicates with a fixed subnet of wireless nodes, resulting in a rigid structure formed with several independent star networks. Thus, the gateways are not coordinated with a protocol that could be employed to improve the dependability of the wireless links.
With the exception of tire pressure monitoring and keyless entry systems, integrated WSNs have not been deployed in cars today, particularly systems employing coordinated resources to avoid message collisions. Future automotive applications are anticipated to be configured with a star topology wireless network, which is generally the better implementation structure for small and resource-limited wireless end devices (nodes). A star topology typically ensures shorter and more predictable message delays. But a disadvantage of a conventional wireless star architecture for such applications is the inhomogeneity of network link quality. Messages transmitted from wireless sensor nodes that are distant from the central unit (CU) or are shielded by reflecting or attenuating materials exhibit a higher BER due to the weaker received signal.
There has also been substantial research in recent years on the medium access control (“MAC”) properties of pure wireless sensor networks intended for autonomous operation in systems deployed over a large physical area. Since wireless communication arrangements are generally susceptible to radio noise and various interference mechanisms, such networks must be properly conditioned to enable their use in uncontrolled industrial and automotive environments. For these and other applications, a combination of wired and wireless communication is a needed alternative.
Past research on medium access control for wireless sensor networks has been primarily aimed at reducing signaling overhead and reducing idle listening time. Most of these projects have been focused on self-organization features and on energy efficiency because previous wireless sensor networks have been optimized for these application issues and requirements.
An example of a typical wireless sensor network application is low data-rate monitoring of a large physical area over an expanded extended period of time using a self-organizing, wireless, multi-hop network. Such networks have been described by A. Mainwaring, et al., in the technical report entitled “Lessons From A Sensor Network Expedition,” University of California, Berkeley and Intel Research Laboratory at Berkeley, 2003, by M. Srivastava, et al., in the technical report entitled “Overview of Sensor Networks, University of California, Berkeley and University of California Los Angeles, August 2004, and by Glaser, S. D., et al., in the paper entitled “Some Real-World Applications of Wireless Sensor Nodes,” Proceedings of the SPIE Symposium on Smart Structures and Materials, NDE 2004, San Diego, Calif., Mar. 14-18, 2004, which documents are hereby referenced and incorporated herein.
In contrast to the networks studied in these research efforts, networks applied in emerging industrial or automotive applications cannot rely on such typical ad hoc wireless communication paths. In automobiles of recent design, many non-safety-critical comfort sensors such as air conditioning sensors and actuators, seat adjustment devices, or tire pressure sensors have been candidates for monitoring with a low data rate wireless sensor network. In previous designs, these sensors and actuators were interconnected with hard wiring, or were not included in the vehicle design.
To ensure adequate response time and data reliability for the more important system elements in a low data-rate network, the local wireless network should be centrally controllable and should ensure deterministic timing (e.g., latency and throughput), at least for high priority sensors and actuators that can be readily identified. Without such assurance, automotive manufacturers will continue to deploy wired sensor networks to meet customer performance expectations.
Thus, there is a need for a new medium access control arrangement for a wireless sensor network that can provide reliable communication for critical network elements and that can ensure a maximum timing latency in view of the network data rate. This need would be satisfied with a communication system that combines wired and wireless communication paths that provide reliable data transfer in a harsh environment.